Lithium-ion batteries as rechargeable high capacity batteries have enabled electronic devices to achieve high performance and long-term operation. Furthermore, lithium-ion batteries, which are installed in vehicles and the like, are the most likely batteries for hybrid and electric vehicles. Currently broadly used lithium-ion batteries have a positive electrode that is prepared by applying a slurry comprising an active material such as lithium cobalt oxide and a binder such as polyvinylidene difluoride (PVDF) to aluminium foil and a negative electrode that is prepared by applying a slurry comprising a carbon-based active material and a binder such as PVDF or styrene-butadiene rubber (SBR) to copper foil.
To further increase the capacity of lithium-ion batteries, use of silicon, germanium, or tin as a negative electrode active material has been studied (see, for example, Patent Document 1). The ability of a negative electrode active material using silicon, germanium, tin, or the like to accept a large amount of lithium ions causes a large volume change in the material between complete charge and complete discharge of batteries. Because a binder such as PVDF, SBR, or the like cannot follow the volume change of the active material, it is contemplated that polyimide resins having more excellent mechanical properties are used as a binder for negative electrodes (see, for example, Patent Document 2). However, these resins can be dissolved only in organic solvents such as N-methylpyrrolidone and N,N′-dimethylacetamide, which causes a problem of a large environmental impact. Thus, use of the resin mixed in an aqueous solvent as an aqueous binder is being studied.
For an aqueous solution of a polyimide resin, an aqueous solution of a polyimide precursor to which an amine is added (see, for example, Patent Documents 3 and 4) and an aqueous mixture solution of a polyimide in which any of hydroxy group, carboxy group and sulfonic group is introduced to side chain positions and a hydroxide of an alkali metal are known (see, for example, Patent Document 5 and Non-Patent Document 1).
For an aqueous solution of a polyamide resin, an aqueous solution of a polyamide resin prepared by polymerization using an excess amount of an acid component rather than an amine component and a resin comprising ammonia or an organic amine is known (see, for example, Patent Document 6).